1. Field of the Invention
This invention relates to a load driving system and method thereof for driving loads using a power source having an output voltage higher than a normal driving voltage of the loads, and more particularly, to an art of simplifying the circuitry and reducing its heat generation amount.
2. Description of the Related Art
The voltage of a battery power source mounted to vehicles such as automobiles and trucks is generally 12 volts or 24 volts. The recent trend is to use a higher battery voltage (52 volts, for example) in order to reduce the value of the current flowing into various circuits or loads mounted to a vehicle. However, such loads as lamps mounted to a vehicle are designed for a conventional battery voltage of 12 volts or 24 volts and cannot be driven when being directly connected to a 52-volts DC power source.
It is thus necessary to replace loads mounted to a vehicle with ones designed for a voltage of 52 volts to match a 52-volts battery voltage. However, such increase in a driving voltage of such lamps as head lamps, tail lamps, brake lamps and room lamps mounted to a vehicle (that is, designing the lamps for 52 volts) results in increase in the size of these devices, causing cost increase, and thus being impractical.
Under these circumstances, various kinds of load driving system for driving loads with a voltage higher than a normal driving voltage without overloading have been proposed and put to practical use. An example of such conventional load driving systems is disclosed in Japanese Patent Laid-Open Publication No. Hei-5-168164 (hereinafter referred to as a xe2x80x9cconventional examplexe2x80x9d).
FIG. 1 is a circuit diagram of a load driving system described in the conventional example. As shown in the figure, the 7 load driving system has a DC power source 101 and an FET 106 directly connected to a lamp 102 as a load. The system further includes a voltage detector 104, a PWM controller 105, and a switch 103.
The PWM controller 105 outputs a pulse signal to the FET 106 to cause it to turn on and off so as to provide a root-mean-square value of the voltage supplied to the lamp 102 smaller than the output voltage of the DC power source 101, thereby preventing the lamp 102 from being overloaded when illuminated.
In the above conventional load driving system, however, the switching operation of the FET 106 based on PWM signals generates switching losses, increasing the amount of heat generated by the element, and resulting in an increased size of a heat sink.
Another method being proposed is shown in FIG. 2, in which a DC/DC converter is used to step down a voltage supplied from a DC power source. Specifically, as shown in the figure, the DC/DC converter has a main switch 110 and an auxiliary switch 111 switched at a desired duty ratio in order to convert the input voltage Vin supplied from the DC power source to a pulsed voltage. The pulse voltage is rectified and smoothed via a circuit consisting of a diode D101, a coil L101 and a capacitor C101 and is output as the output voltage Vout.
The DC/DC converter thus using the auxiliary switch 111 and a resistor R101 connected to the switch 111 has an advantage of reduced switching losses as compared with only performing PWM control. However, the DC/DC converter continuously outputs a low voltage and requires the coil L101 and the capacitor C101 for use as a filter. This disadvantageously complicates the circuitry. Further, there is a problem of heat generated by the coil L101 and the capacitor C101.
When a component constituting part of the DC/DC converter cannot be surface-mounted, heat generated by the component is dissipated inside a case housing the component, increasing the entire temperature inside the case.
Further, power is supplied even when the main switch 110 is off, which prevents lowering the operating frequency of the main switch 110.
It would be conceived to provide an auxiliary switch similar to the auxiliary switch 111 shown in FIG. 2 to each switching element (FET 106 shown in FIG. 1) for driving loads so as to reduce switching losses. This method, however, increases the number of switching elements, resulting in entire cost increase, and thus being impractical.
As described above, the conventional load driving system shown in FIG. 1 disadvantageously causes great switching losses. The DC/DC converter shown in FIG. 2 has a large amount of heat generated from the components and cannot be lowered in the operating frequency.
This invention was made to solve the above conventional problems, and has an object of providing a load driving system capable of reducing losses and noise associated with the switching operation and also reducing the amount of heat generation.
According to a first technical aspect of this invention, there is provided a driving system connected to a DC power source which supplies a predetermined output voltage, for driving at least one load, which has a voltage converter having a first switch for converting a voltage outputted from the power source to a pulse voltage having a desired duty ratio for output, the first switch generating the pulse voltage in response to a first control signal; and a second switch connected between the voltage converter and the load, the second switch turning on and off in response to a second control signal.
According to a second aspect of this invention, there is provided a method for driving at least one load using a DC power source supplying a predetermined output voltage, which has the steps of converting a voltage outputted from the power source to a pulse voltage having a desired duty ratio for output in response to a first control signal; and selectively switching between conductive and non-conductive states of the pulse voltage to the load in response to a second signal, wherein, to make the load in a conductive state, the second control signal is outputted to switch from the non-conductive state to the conductive state during an off state of the pulse voltage.